Orthogonal frequency division multiplexing (OFDM) is a digital transmission technique in which a signal is split into a series of orthogonal narrowband subcarriers. Each subcarrier can be modulated using any of a variety of modulation techniques (e.g. quadrature amplitude modulation (QAM), binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), etc.), and an inverse Fourier transform may be performed on the subcarriers. The subcarriers typically are uncorrelated to each other, and phase differences between the subcarriers may be random. In consequence, large variations in the instantaneous power level of the signal can occur. For example, when the phases of all the subcarriers combine constructively, the power level of the signal may peak well above the average power level. This large peak to average power ratio (PAPR) is characteristic of OFDM systems.
A disadvantage of a signal having a large PAPR is that complex analog-to-digital and digital-to-analog conversion may be required to process the signal due to a large number of bits that are required to represent the signal. Another disadvantage is that power amplifiers with large linear operating ranges may be required for signal processing. Power amplifiers having large linear operating ranges typically are not efficient because large bias currents are necessary to deliver the occasional power peak. This issue can lead to a requirement for power detection and power reduction schemes for the power amplifier, which consume valuable system resources. Accordingly, it is desirable to reduce OFDM signal PAPR.
There are three categories of techniques that are sometimes implemented to reduce PAPR. One category of techniques relies on signal distortion. Signal distortion can be implemented, for example, by nonlinearly manipulating the OFDM signal at or around the signal peaks, which results in reduced PAPR. The simplicity and efficiency of this technique contributes to its wide use, but leads to signal damage, and therefore produces a higher symbol error rate (SER). The second category uses encoding methods that exclude OFDM symbols with large PAPR, and the third category of techniques scramble each OFDM symbol with different sequences. The last two techniques require that both transmitter and receiver perform paired signal processing, a methodology not supported by some communication system standards.